EFFECT OF THE N-TERMINAL GLYCINE ON THE SECONDARY STRUCTURE, ORIENTATION, AND INTERACTION OF THE INFLUENZA HEMAGGLUTININ FUSION PEPTIDE WITH LIPID BILAYERS

The amino-terminal segment of the membrane-anchored subunit of influen za hemagglutinin (HA) plays a crucial role in membrane fusion and, hen ce, has been termed the fusion peptide. We have studied the secondary structure, orientation, and effects on the bilayer structure of synthe tic peptides corresponding to the wild-type and several fusogenic and nonfusogenic mutants with altered N-termini of the influenza HA fusion peptide by fluorescence, circular dichroism, and Fourier transform in frared spectroscopy, All peptides contained segments of alpha-helical and beta-strand conformation. In the wild-type fusion peptide, similar to 40% of all residues were in alpha-secondary and similar to 30% in beta-secondary structures, By comparison, the nonfusogenic peptides ex hibited larger beta/alpha secondary structure ratios. The order parame ters of the helices and the amide carbonyl groups of the beta-strands of the wild-type fusion peptide were measured seperately, based on the infrared dichroism of the respective absorption bands. Order paramete rs in the range 0.1-0.7 were found for both segments of the wild-type peptide, which indicates that they are most likely aligned at oblique angles to the membrane normal, The nonfusogenic but not the fusogenic peptides induced splitting of the infrared absorption band at similar to 1735 cm(-1), which is assigned to stretching vibrations of the lipi d ester carbonyl bond, This splitting, which reports on an alteration of the hydrogen bonds formed between the lipid ester carbonyls and wat er and/or hydrogen-donating groups of the fusion peptides, correlated with the beta/alpha ratio of the peptides, suggesting that unpaired be ta-strands may replace water molecules and hydrogen-bond to the lipid ester carbonyl groups. The profound structural changes induced by sing le amino acid replacements at the extreme N-terminus of the fusion pep tide further suggest that tertiary or quaternary structural interactio ns may be important when fusion peptides bind to lipid bilayers.